linux-kernel - Re: preempt bug in set_pmd

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Message-ID: <20080305064814.GB28398@elte.hu>
Date:	Wed, 5 Mar 2008 07:48:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: preempt bug in set_pmd_pfn?


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Ingo Molnar wrote:
>> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>>   
>>> I think set_pmd_pfn, which is only called by __set_fixmap, might have a 
>>> preempt bug in it.
>>>     
>>
>> yes, and we had similar preemption bugs in the past. I guess most places 
>> are either infrequent or have some natural atomicity anyway. Wanna send a 
>> patch?
>
> Sure.  Should it just disable preemption, or take a lock?  It calls 
> set_pte_at without holding any pte locks; that seems to be relatively 
> common.  Is it OK when you're operating on init_mm?

no, it's not OK to modify the kernel pagetable without locking - taking 
the pgd_lock should do the trick. Could you send the stacktrace that 
shows the place that is preemptible?

	Ingo
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